WO1993001331A2

WO1993001331A2 - Anodes for cathodic protection

Info

Publication number: WO1993001331A2
Application number: PCT/GB1992/001274
Authority: WO
Inventors: Thomas Gallacher
Original assignee: Jennings Winch & Foundry Co Ltd
Current assignee: Jennings Winch & Foundry Co Ltd
Priority date: 1991-07-12
Filing date: 1992-07-13
Publication date: 1993-01-21
Anticipated expiration: 1994-01-12
Also published as: WO1993001331A3; GB9115184D0

Abstract

An anode (20) for protecting a reinforced concrete structure by impressed-current cathodic protection is formed of two sheets (21, 22) of polymer matrix such as polypropylene having a filler of Fe-Ti, TiOx or TiO2 or mixtures thereof to render the anode conductive. The anode may be in the form of a tile formed from two sheets (21, 22) with an electrical conductor (23) therebetween. The tile has apertures (26) to permit concrete or grout to bond to an underlying concrete surface (33).

Description

Anodes for Cathodic Protection

The present invention relates to anodes for use in

impressed-current cathodic protection of embedded steel elements such as reinforcing bars in concrete structures.

The widespread use of de-icing substances, such as common salt, has resulted in the need for cathodic protection of reinforced concrete structures such as road bridges and multi-storey car parks.

There have already been proposals for composite anodes for use in such impressed-current cathodic protection systems for reinforced concrete structures. Thus, EP-A-0 085 582 discloses an anode for use in such a system consisting of a matrix of synthetic plastics material loaded with a particulate carbon material in the form of graphite or petroleum coke to provide the required electrical conductivity.

Tests which we have had carried out show, however, that although providing good conductivity graphite showed poor resistance to saline solution when used as an anode. The graphite was rapidly attacked and the resin matrix also exhibited some signs of sponginess.

EP 0 186 334 discloses a porous anode formed of TiO_x in bulk form where x is in the range 1.6 to 1.9, preferably 1.67 to 1.85.

An anode according to the present invention is characterised in that the particulate filler incorporated in a polymer matrix is Ferro Titanium, TiO_x or a misture thereof.

Typically, the Titanium content of the Ferro Titanium will be in the range 50-80% by weight of the Ferro Titanium. Preferably, the Ferro Titanium particles are in the size range of 300-500 microns.

A suitable synthetic plastics resin matrix is an epoxy resin available from Ciba Geigy under the specification CY219 ARALDITE (Trade Mark) resin and HY219 hardener, together with an appropriate accelerator. When first mixed, this resin composition has a low viscosity enabling a relatively large proportion of Ferro Titanium particles to be incorporated into it. However many other polymers are suitable, such as polypropylene and synthetic and natural elastomers.

Initial tests were carried out on a variety of materials as potential conductive fillers. The materials used were:

Graphite - 100% electro conductive graphite powder.

Ferro Silicon 50% and 75% concentrations of Si,

Ferro Chromium 57% concentration of Chromium.

Fe Titanium - 70% concentration of Ti.

Fe Niobium - 70% concentration of Nb.

Simple mixtures of the resin and the metal particles were made using the resin system designed to take maximum material loading. Mixtures of 50:50 by weight, or the maximum possible were produced.

The results were:

Graphite Good condition - poor resistance to saline solution when used as an anode. The graphite was rapidly attacked and the resin also exhibited some signs of sponginess.

Ferro Silicon Very poor conduction. Not suitable. Ferro Chromium Very poor conduction. Not suitable. Ferro Titanium Moderate conduction, Good resistance to chemical resistance.

Ferro Niobium Moderate conduction, Poor resistance to

chemical corrosion.

In use, the anode may be formed by pouring the mixture of polymer and metal particles into a depression formed in a horizontal concrete surface and subsequently concreting over the anode thus formed, after providing a suitable electrical connection to the anode. A further electrical connection is made to the

reinforcement steel within the concrete so that the steel acts as cathode when an appropriate voltage is applied between the anode and cathode to form an impressed current cathodic protection system.

Where the anodes are to be applied to vertical or sloping surfaces, it is generally more convenient to preform the anodes as tiles, for example about 2 to 3mm thick. These can then be concreted into the structure to be protected.

The invention will now be further described by way of example with reference to the drawings, in which:

Figures 1 to 3 are graphs showing variation of impressed current with time;

Figure 4 is a plan view of an anode; and

Figure 5 is a longitudinal sectional view of part of a reinforced concrete structure incorporating the anode of figure 4, the anode being shown in section on the line V-V of figure 4.

Figure 1 shows the variation of impressed current with time for anodes containing different percentages by weight of Ferro Titanium particles (in all cases consisting of about 71% by weight of Titanium ) .

The preliminary results (from 12" cube concrete blocks) showed that the current received at the cathodic dropped sharply over a short period of time when standing in air. This is thought to be due to the migration of protective ions towards the cathode (the mechanism of current conduction) passivating the steel. The observed drop in current is not thought to be due to failure of the anode. The cubes were then placed in a saline solution as it was though that the salt ions present would break down the passivating layer allowing current passage to resume. However, due to the size of the 12" cubes, little breakdown was observed and the current passed was low (see Figs. 2 and 3). Again this is thought to be due to the concrete and the mechanisms of conduction in concrete rather than anode failure.

Work then took place using a 4" cube specimen. The specimen has conducted a current, without failure, for over fifty days under a high driving voltage, and corresponding current, to give

accelerated testing. It has been observed that attack has occurred on the anode either by attack on the concrete/anode interface or through the protective resin covering placed over the anode. The attack is observed as rust coloured growths on the surface of the protective covering. At the time of writing this attack has had no deleterious effects.

Resin/filler tiles have been manufactured and have a relatively low resistance (10-20 ohms) although none have so far been tested. The resin tiles however do overcome the problems of the placement of the anode to other than horizontal surfaces and sinking of the conductive filler. Table 1 - Range of Metal Alloy Fillers

Filler Conduction? Min Voltage for Weight Loss (Kg/A/Yr)

Conduction

FeTi Yes 1.4 V 5.2

FeNb Yes 1.6 V 5.9

FeSi Yes 1.2 V 18.2

FeCr No -

Table 2 - Effect of FeTi Volume Fraction

Volume Conduction? Min Voltage for Weight Loss (Kg/A/Yr)

Fraction Conduction

24% Yes 1.8 V -

39% Yes 1.6 V 5.3

48% Yes 1.2 V 3.2

52% Yes 1.4 V 6.1

56% Yes 1.4 V 6.1

Table 3 - Effect of Particle Size {39% FeTi)

Particle Conduction? Min Voltage for Weight Loss (Kg/A/Yr)

Size Conduction

(Microns)

500-353 Yes 0.6 V 4.3

353-251 Yes 0.6 V 3.8

251-152 Yes 0.8 V

152-104 Yes 0.6 V

104-76 Yes 1.0 V

76-53 No - -

53- No - - Figures 4 and 5 show in plan and longitudinal section respectively a practical form of anode tile embodying the invention. The tile consists of two sheets 21 and 22 of a polymer such as polypropylene containing 50 to 60% by volume of a filler consisting of

Ferro-Titanium or TiO_x or a mixture of these two substances, in particular form, the maximum size of the filler particles being 150 microns. The polypropylene may for example be Neste VC35 12H (polypropylene homopolymer for injection moulding, MFI 35). Each sheet may be formed by "picture frame" compression moulding or by extrusion or rolling and thereafter cutting into sheets of appropriate dimensions for easy handling and installation. The surfaces of the sheets are abraded (with emery paper, grade 60 to 200, or by wire brushing) to remove any skin of polymer on the surfaces. At least one electrical conductor 23 is placed on the top surface of- the lower sheet 22, the top sheet 21 is placed on top and suitably bonded to the lower sheet. For smaller tiles it may be sufficient to have one straight conductor 23 as shown in figure 4. If desired, however, a greater number of conductors may be provided or one (or more) conductor may be used which follows a sinuous path as indicated at 25 in broken lines in figure 4.

The resulting tile 20 is formed with a large number of apertures 26 extending through both sheets 21 and 22, the area of the apertures 26 being for example in the range of 40 to 60%, typically 50%, of the total surface area of the tile.

In use, for protection of a concrete structure 31 (figure 5) having reinforcing steel bars 32, the surface 33 of the structure 31 is first cleaned (and repaired as necessary) and tiles 20 are fixed to the surface 33. The tiles may be laid close to each other so is effectively to cover the whole of the surface 33 and their

conductors 23 are connected together. A concrete layer 34 or grout is then applied over the tiles and fills the holes 26 to bond with the surface 33. The tiles may be for example 2mm thick.

The amount of filler should be such that the electrical resistivity should be not greater than one ohm cm. Typical impressed current density should be in the range 20 to 40 mA/m², the maximum operating density being 100 mA/m² for periods up to three months.

Claims

1. An anode (20) for use in impressed current cathodic protection, comprising a polymer matrix incorporating sufficient electrically conductive particulate filler to render the anode conductive, wherein the particulate filler comprises at least one titaninium alloy or compound.

2. An anode according to claim 1, wherein the filler comprises Ferro-Titanium, TiO₂ or TiO_x where x lies in the range of 1.3 to 2.0, or a mixture of two or more of these particulate materials.

3. An anode according to claim 2, wherein the maximum particle size of the filler is 150 μm.

4. An anode according to claims 2 or 3, wherein the particle content is at least 35%, preferably 50 to 60% by volume.

5. An anode according to any preceding claim, comprising two sheets secured together with an electrical conductor therebetween.

6. An anode according to any preceding claim, in sheet form having apertures therethrough distributed over the surface of the sheet anode.

7. An. anode according to claim 6, wherein the apertures comprise 40 to 60% of the sheet area.